It is recognized by those of skill in the relevant art that phosphorus is a critical nutrient to the growth of plants, and increases crop yields. Thus, efforts have been made to produce fertilizer which increases the amount of soluble phosphate in soil. One such method involves performing a variety of chemical steps. In particular, ground phosphate rock is mixed with concentrated sulfuric acid. This reaction forms green phosphoric acid (30%) and gypsum (CaSO.sub.4.cndot.2H.sub.2 O) slimes. The green phosphoric acid is then filtered from these slimes, and concentrated to about 60%. The concentrated phosphoric acid is then reacted with ground phosphate rock particles to form triple superphosphate (TSP), which is then used as fertilizer. The evaporation of the phosphoric acid gives rise to significant quantities of hydrofluoric acid which pollutes the atmosphere.
However, the chemical production of phosphate fertilizer from sulfuric acid has numerous drawbacks. Initially, the process involves the use of containment ponds of concentrated sulfuric and phosphoric acid. Acid from these ponds can leach into ground water supplies and cause extensive environmental damage. Furthermore, the disposal of waste products from the TSP production process can be expensive. In particular, numerous gypsum slimes are produced as a byproduct of TSP production. These slimes can escape from containment ponds and enter the aquifer, polluting regional water supplies. What's more, other byproducts of the process, such as ammonium and potassium phosphates, can drain into nearby wells, lakes and streams. In those waters, eutrophication processes can occur, which pollute the water and place great strains on the ecosystem.
In order to avoid such environmental damage, efforts have been made to utilize indigenous microorganisms of soil to produce phosphate for consumption by crops. In particular, numerous microorganisms, particularly of the genus Thiobacillus are found in soil which oxidize elemental sulfur into sulfuric acid. Phosphate sulfur fertilizer has been developed to exploit the action of these microorganisms. One such composition involves a mixture of finely-powdered bone or mineral phosphate and finely-powdered sulfur, which are mixed together in a dry state. This mixture is then spread over the land. The Thiobacillus microorganisms oxidize the sulfur to produce sulfuric acid. The sulfuric acid in turn reacts with the phosphate to produce soluble phosphates in the soil. However, the use of this mixture has met with only limited success. In particular, it has been discovered that in order to utilize these microorganisms, contact between the sulfur and the mineral phosphate must be maintained. Since the sulfur and phosphate can be separated in the application of this mixture, contact between the sulfur and phosphate can be lost. As a result, the sulfuric acid produced by the Thiobacillus does not react with the phosphate, and soluble phosphates are not produced.
Another composition utilizing sulfur oxidizing microorganisms in soil involves mixing ground phosphate mineral with molten sulfur to form a slurry. This slurry is then cooled so that it solidifies. The solid is then ground to produce particles containing sulfur and phosphate particles. These fertilizer particles are then spread on soil, where Thiobacillus microorganisms oxidize the sulfur as described above. However, this type of phosphate fertilizer also suffers from limitations. In particular, these types of fertilizers consist mainly of phosphate rock particles coated with sulfur. Thus, even though Thiobacillus microorganisms are able to form sulfuric acid from this sulfur coating, the sulfuric acid produced is unable to react with the phosphate rock until the entire coating of sulfur has been removed. As a result, the reaction between the sulfuric acid and phosphate rock particles is poor, and the amount of soluble phosphate delivered to plants with this type of fertilizer is low.
Accordingly, what is needed is a novel phosphate fertilizer particle which exploits the function of indigenous microorganisms, but permits phosphate rock particles to be readily available for reaction with sulfuric acid produced by such microorganisms. As a result, the production rate of soluble phosphate for use by crops is increased for a given period of time.
What is also needed is novel method for producing a phosphate fertilizer particle wherein the phosphate is not completely sealed within sulfur, such that phosphate is available for immediate reaction with sulfuric acid produced by indigenous Thiobacullus milcroorganisms.
The citation of any reference herein should not be construed as an admission that such reference is available as "Prior Art" to the instant application.